Contemporary multichannel radio frequency (RF) communication systems typically employ one or more channels to pass signalling and control information to and between subscriber units. In frequency division multiple access (FDMA) systems, each communication channel occupies one particular frequency within an allocated bandwidth. Since bandwidth is constrained, there are only a limited number of frequency channels available for communication with sufficient frequency separation to avoid co-channel interference, e.g. one communications channel per 25 KHz. More efficient bandwidth use is achieved in Time Division Multiple Access (TDMA) systems in which each suitably separated frequency is divided into a number of distinct time slots, each time slot forming a separate channel for communication. Accordingly, several communications channels are formed on a single frequency using time division multiplex principles. Although somewhat more complicated, TDMA systems considerably increase capacity of the allocated bandwidth, e.g. three communications channels per 25 KHz, and for that reason are desirable.
The present invention contemplates a trunked, digitized speech (FDMA or TDMA) communications system with half-duplex digital radios operating to transmit or receive information (but not both) at any particular instant. Compared to prior art half-duplex trunked radios which send continuous analog speech signals over specific frequency channels, digitized speech systems incur additional time delays because of speech processing operations required. For example, unavoidable delays are caused by (1) speech coding frame and pipeline architectures; (2) signal processing, e.g. analog-to-digital conversion, encoding, encrypting, and transmitting speech and control signals; (3) signal routing through a repeater/site controller; and (4) signal decoding, decrypting, and digital-to-analog conversion at the receiving radio.
In TDMA systems, delays are further exacerbated since each channel transmission occurs in a specified time slot on a particular frequency. As a result, data and speech must be buffered and stored before transmission until the proper time slot occurs. Obviously, these delays can be a source of inconvenience and annoyance to users both at initiation and conclusion of conversations.
For example, annoying delays occur after users "disconnect" from a communications channel. When a first radio operator finishes speaking and releases his push-to-talk (PTT) button, a second listening radio operator reasonably expects that he can respond immediately. However, processing and buffering delays require the second operator wait a predetermined time period before his radio accepts speech inputs.
After the first operator releases his PTT button, a significant time period expires before a "disconnect signal" is transmitted via radio link to the trunked repeater/site controller system. That time period allows currently and recently accepted speech to be processed and transmitted. Soon after receiving the disconnect signal, a signal is transmitted from the controller system to all "second" operators in a group listening on that particular channel to drop the current working channel, switch to a control channel, and await assignment of a new working channel. A new working channel is assigned to a second user who desires to speak only after that time period has expired. Clearly, it is desirable to have a signalling strategy for minimizing the effect of the unavoidable time delays associated with channel disconnect in FDMA and TDMA trunked systems.
Similar delay problems affect system access. For example, when a user wants to communicate over the trunked system, he depresses the radio PTT button to transmit a request for a working channel over the assigned control channel. The repeater/site controller system responds with a "channel access signal" which assigns a working frequency within the allocated bandwidth (accompanied by a specific time slot in TDMA systems). The user's radio then emits an alert tone indicating that conversation may be commenced.
A significant time delay exists between receipt of a channel access signal and generation of an alert tone signalling that the radio will now accept speech input. That delay is caused by necessary radio "set up" procedures, e.g., switching radio frequency synthesizers from the control frequency channel to the working frequency channel, synchronizing the allocated time slot channel (in TDMA systems), loading significant system and feature related software into its signal processor, adjusting the RF power, etc. Accordingly, it would be desirable to develop a signalling and processing system which minimizes delays in disconnect from and access to a radio channel in a digitized radio communication system.
The present invention advantageously resolves the above-described problems in half-duplex, trunked digital communications between mobile transceivers by providing a means for minimizing time delays occurring after one transceiver ends a transmission and before another transceiver communication may commence. Such minimizing means may include means for transmitting a disconnect signal from the currently transmitting transceiver during a time frame immediately following indication of an end to a communication and/or means for permitting speech input to a transmitting transceiver immediately upon receipt of a communications channel.
The system of the present invention has particular applicability to time division multiple access (TDMA) communications systems where each time frame is divided into plural time slots. For channel disconnect, the disconnect signal is transmitted during a first time slot in a time frame immediately after detecting a change in the status of a transmission switch on the transceiver. For channel access, an alert tone is generated immediately during a time slot when a communications channel has been assigned. However, speech captured in subsequent time slots in that time frame is not transmitted.
The present invention further includes a method for minimizing channel disconnect delays in a half-duplex radio system including plural mobile transceivers for processing speech in consecutive time frames. Included are steps of (a) detecting completion of a transceiver transmission in a first time frame; (b) transmitting a disconnect signal during a second immediately following time frame; and (c) prohibiting transmission of speech captured in the first time frame unless there is sufficient processing time before the second immediately following time frame. Before step (a), the present invention may include the additional steps of capturing speech in consecutive time frames, and processing and storing speech in time frames immediately following the time frame of capture. Time frames may be divided into a series of time slots with each time slot providing a separate communications channel. Accordingly, the disconnect signal is transmitted in step (b) during the speaker's first available assigned time slot in the second time frame.
After step (a) and during the first time frame, the present invention includes the steps of processing and storing the speech captured in the immediately preceding time frame, and transmitting the processed and stored speech during the second time frame. The disconnect signal is transmitted along with the processed and stored speech from the preceding time frame over a working communications channel.
The present invention also includes a method for minimizing channel access delays in the half-duplex radio system including plural mobile transceivers for processing speech in consecutive time frames including the steps of (a) receiving a working channel via a control channel during the first time frame; (b) immediately thereafter generating an alert signal to a transceiver user and permitting receipt of speech input from the user; and (c) processing and transmitting speech received in consecutive time frames.
Before step (a) the present invention may additionally include the steps of depressing a transceiver transmit switch and generating a channel access request over the control channel. Simultaneous with alert signal generation, the method of the present invention includes performing speech processing and transmission functions, and thereafter, performing functions relating to tasks other than speech processing and transmission.
These as well as other advantages, objects and features of the invention will be better appreciated by a careful study of the following detailed description of the presently preferred exemplary embodiment of this invention in conjunction with the accompanying drawings, of which:
FIG. 1 is a schematic block diagram of the overall hardware architecture of a mobile transceiver in a trunked radio system which may be utilized to implement the present invention;
FIG. 2 is a frame diagram illustrating channel disconnect procedure according to the present invention in the context of a TDMA system;
FIG. 3 is a frame diagram illustrating a channel access procedure according to the present invention in the context of a TDMA system;
FIG. 4 is a schematic flow chart diagram of exemplary program steps relating to channel disconnect procedures according to the present invention; and
FIG. 5 is a schematic flow chart diagram of exemplary program steps relating to channel access procedures according to the present invention.